Many examples of tellurium-containing catalyst are known, and they include a catalyst of the type described in U.S. Pat. No. 3,153,085 which is made of an oxide containing molybdenum, cobalt, and tellurium, a catalyst of the type described in Japanese Patent Publication No. 7774/66 which is made of an oxide containing molybdenum, zinc, and tellurium, a catalyst of the type described in U.S. Pat. No. 3,446,834 which is made of an oxide containing tellurium and cerium, a catalyst of the type described in U.S. Pat. No. 3,668,147 which is made of an oxide containing antimony, vanadium, molybdenum, tungsten and tellurium, and a catalyst of the type described in U.S. Pat. No. 3,641,102 which is made of an oxide containing molybdenum, iron and tellurium. These catalysts are known to be useful for oxidation of olefins and ammoxidation. Catalysts of the type described above which are made of metal oxides are produced from compounds of the respective metals which are thermally decomposed to oxides, and the nature of this method is such that the compounds used as the starting material are desirably soluble in solvents, particularly in water. The patents mentioned above obtain the tellurium component from metallic tellurium, tellurium dioxide, telluric acid, etc. But an aqueous solution containing tellurium is not easy to prepare from metallic tellurium or tellurium dioxide. In the patents cited above, metallic tellurium and tellurium dioxide are directly mixed with the other catalyst components.
U.S. Pat. No. 4,049,575 describes a method for producing a high-performance antimony-polyvalent metal oxide catalyst useful for the oxidation or ammoxidation of hydrocarbons. The method comprises calcining an antimony polyvalent metal oxide composition used as the catalyst base, impregnating the composition with a solution or suspension containing tellurium and other additives, drying the composition, and finally calcining the dried composition. However, this method has several problems particularly with the preparation of the solution containing tellurium and other additives, and those problems present difficulties in commercial application of the method. In the method of U.S. Pat. No. 4,049,575, and aqueous solution containing tellurium is prepared by a first technique comprising dissolving metallic tellurium, tellurium dioxide, or tellurous acid in nitric acid, or by a second technique comprising dissolving telluric acid in water. But the first technique requires much nitric acid to dissolve the tellurium compound, and because of low solubility of tellurium, a tellurium containing precipitate is formed at low concentration of nitric acid. In addition, the resulting tellurium-containing solution is low in miscibility with other catalyst components such as vanadium, molybdenum, and tungsten. If compounds of these components are added to the tellurium-containing solution, a precipitate results in most cases. Hence, it has been very difficult to produce by the first technique a homogeneous solution of catalyst components that is stable to any concentration. Also, the use of nitric acid presents other problems, such as corrosion of the reaction vessel and air pollution by NO.sub.x in the waste gas. A stable aqueous solution containing tellurium is easily prepared by the second technique of dissolving telluric acid in water, because telluric acid has great solubility in water and does not have a tendency to form readily a precipitate in the presence of other catalyst components. However, telluric acid is conventionally produced by a process involving many refining steps, including oxidation of metallic tellurium with chloric acid or oxidation of tellurium dioxide with potassium permanganate, and, since telluric acid does not find utility in many industrial uses, it is extremely expensive and is therefore not economically suitable for use as a material in commercial production of catalysts. Another problem is that tellurium dioxide and telluric acid of high purity are not readily available, and this is perhaps due to the nature of the processes for making them. As described above, none of the conventional materials for use as a tellurium catalyst component are satisfactory, but since metallic tellurium of relatively high purity is easily available at lower cost than telluric acid, it would be very beneficial metallic tellurium could be as a catalyst component. However, as already mentioned, dissolving metallic tellurium in nitric acid involves three problems, i.e much nitric acid is necessary, the resulting solution is relatively unstable, and it is not highly miscible with other catalyst components. Use of much nitric acid is desirably avoided because it presents other problems such as corrosion of the reaction vessel and air pollution by NO.sub.x in the waste gas.
It is known that a slight amount of metallic tellurium dissolves in hydrogen peroxide at a very slow rate (see J. Less Common Metals, Vol. 16, pp. 215-222, 1968), but such a slow reaction rate is impractical. This problem can be solved to some extent by using a powder of metallic tellurium according to the common practice in improving the rate of solid-liquid reaction by increasing the surface area, but the result is still unsatisfactory. Also, a method is known to react metallic tellurium with hydrogen peroxide in the presence of nitric acid (German Patent Application (OLS) No. 2,041,842), but since the presence of nitric acid is essential to this method, corrosion of the reaction vessel and air pollution by NO.sub.x are unavoidable.
To solve the problems of the conventional techniques for preparing a tellurium-containing solution from metallic tellurium, and particularly for use in producing a tellurium-antimony containing oxide catalyst, various studies have been made, resulting in accomplishment of the invention described herein.